Elastic scattering of low-energy electrons by benzene

Experimental and theoretical analysis for total electron scattering cross sections of benzene

Measurements of the total electron scattering cross sections (TCSs) from benzene, in the impact energy range of 1-1000 eV, are presented here by combining two different experimental systems. The first utilizes a magnetically confined electron transmission beam for the lower energies (1-300 eV), while the second utilizes a linear transmission beam apparatus for the higher energies (100-1000 eV). These cross sections have also been calculated by means of two different theoretical methods, the Schwinger Multichannel with Pseudo Potential (SMCPP) procedure, employing two different approaches to account for the polarization of the target for impact energies between 0.1 and 15 eV, and the Independent Atom Model with the Screening Corrected Additivity Rule including Interference effect (IAM-SCAR+I) paradigm to cover the 10-10 000 eV impact energy range. The present results are compared with available theoretical and experimental data, with the level of accord being good in some cases and less satisfactory in others, and some predicted resonances have been identified. In particular, we found a π^* shape resonance at 1.4 eV and another feature in the energy region 4.6-4.9 eV interpreted as a π^* resonance (²B_2g symmetry), which is a mixture of shape and a core excited resonance, as well as a Feshbach resonance at 5.87 eV associated with the 3s (a_1g) Rydberg state. A Born-type formula to extrapolate TCS values for energies above 10 000 eV is also given. This study provides a complete set of TCS data, with uncertainty limits within 10%, ready to be used for modeling electron transport applications.

Shape resonances, virtual state, and Ramsauer-Townsend minimum in the low-energy electron collisions with benzene

In this work, we revisit the low-energy electron scattering by benzene. We employed the Schwinger multichannel method implemented with pseudopotentials to carry out systematic cross section calculations with different schemes of polarization for the resonant and the totally symmetric irreducible representations within the D_2h symmetry group. We present integral and differential cross sections for incident electron energies up to 12 eV and discuss the shape resonances and the presence of a Ramsauer-Townsend minimum and a virtual state in the former. We also discuss the relation of these physical phenomena with the different schemes of the polarization effects employed in our calculations. Finally, the comparison of our calculated integral and differential cross sections with the available data from the literature suggests improvement in the agreement between theory and experiment.

Collisions of low-energy electrons with cyclohexane

We report calculated cross sections for elastic scattering of low-energy electrons by cyclohexane (c-C6H12). We employed the Schwinger multichannel method implemented with norm-conserving pseudopotentials in the static-exchange and static-exchange plus polarization approximations, for impact energies up to 30 eV. We compare our calculated integral cross section with experimental total cross sections available in the literature. We also compare our calculated differential cross sections (DCSs) with experimental results for benzene and experimental and theoretical results for 1,4-dioxane, in order to investigate the similarities between those molecules under electron collisions. Although benzene is a cyclic six-carbon molecule, as cyclohexane, we found that the differential cross sections of the latter are more similar to those of 1,4-dioxane than those of benzene. These similarities suggest that the geometry may play an important role in the behavior of the DCSs of these molecules. Our integral cross section displays a broad structure at around 8.5 eV, in agreement with the total cross section experimental data of 8 eV and vibrational excitation data of 7.5 eV. The present integral cross section also shows the presence of a Ramsauer-Townsend minimum at around 0.12 eV. In general, our integral cross section shows a qualitative agreement with the experimental total cross section.

Elastic scattering of low-energy electrons by 1,4-dioxane

We report calculated cross sections for elastic collisions of low-energy-electrons with 1,4-dioxane. Our calculations employed the Schwinger multichannel method with pseudopotentials and were carried out in the static-exchange and static-exchange plus polarization approximations for energies up to 30 eV. Our results show the presence of three shape resonances belonging to the Bu, Au, and Bg symmetries and located at 7.0 eV, 8.4 eV, and 9.8 eV, respectively. We also report the presence of a Ramsauer-Townsend minimum located at around 0.05 eV. We compare our calculated cross sections with experimental data and R-matrix and independent atom model along with the additivity rule corrected by using screening coefficients theoretical results for 1,4-dioxane obtained by Palihawadana et al. [J. Chem. Phys. 139, 014308 (2013)]. The agreement between the present and the R-matrix theoretical calculations of Palihawadana et al. is relatively good at energies below 10 eV. Our calculated differential cross sections agree well with the experimental data, showing only some discrepancies at higher energies.

Stable negative ions and shape resonances in a series of organic molecules

We report on the theoretical determination of low-lying shape resonances in a selected set of seven molecules. The finite element discrete model method is used and the absolute differences between calculated and experimental values, where known, are ≲0.15 eV for the resonances lowest in energy. Difficulties expected with the higher calculated values are discussed. This article reports results for ortho-benzyne, benzene, naphthalene, anthracene, styrene, formamide, and acetamide. Comparisons are made with a few other calculations, again where available.

Electron scattering cross sections from anthracene over a broad energy range (0.00001-10,000 eV)

We report a computational investigation of electron scattering by anthracene (C14H10) in the gas phase. Integral and differential cross sections have been calculated by employing two distinct ab-initio quantum scattering methods: the symmetry adapted-single centre expansion method (ePOLYSCAT) and a screening corrected form of the independent atom model (IAM-SCAR) at low and high energies, respectively. After a detailed evaluation of the current results, we present a complete set of integral scattering cross sections from 0.00001 to 10,000 eV.

Resonant channel coupling in electron scattering by pyrazine

Detailed investigation of the three low-energy resonances seen in electron scattering by the diazabenzene molecule pyrazine reveals that the first two are nearly pure single-channel shape resonances, but the third is, as long suspected, heavily mixed with core-excited resonances built on low-lying triplet states. Such resonant channel coupling is likely to be widespread in pi-ring molecules, including the nucleobases of DNA and RNA, where it may form a pathway for radiation damage.

Low-energy electron diffraction and resonances in DNA and other helical macromolecules

We propose a framework to calculate the intermolecular multiple elastic scattering of low-energy electrons from helical macromolecules and indicate how it affects the resonant capture cross section. Using a model of DNA, an appreciable enhancement of the elastic and resonant capture cross sections is predicted at incident energies below 15 eV. These results may qualitatively explain the observed prominence of low-energy resonances in strand breaking of plasmid DNA.